The present invention relates to projection cathode ray tubes, and more particularly, to a fluid cooled arrangement for mounting the target member of the tube to promote efficient heat transfer from the target member, and thereby enhance the performance and extend the life cycle of the tube.
The use of projection cathode ray tubes to project electronically generated images onto remotely located viewing screens is well known. An example of the construction of a projection tube may be seen in U.S. Pat. No. 4,177,400 to Herhenrother et al., issued Dec. 4, 1979. As described in the U.S. Pat. No. 4,177,400, a beam of electrons emitted by an electron gun is directed toward a curved target member coated with an image producing material, which typically comprises one or more fluorescing phosphors. The resulting image is reflected by a facing mirror out through a correction lens onto the remote screen for viewing. The supporting structure for the target member permits the axial position and tilt of the target member to be adjusted to focus the image of the tube.
A substantial effort has been expended to provide improved tubes for projecting clear, bright and well focussed images needed for many applications, such as in flight simulators, where the objective is to create as close as possible a live training environment. To provide such images it is frequently necessary to operate the tube with sustained electron beams of high intensity impacting against the image forming phosphor coating. Energy from the beam is absorbed by the phosphor coating with only a fraction being released in the form of visible light. The balance of the energy becomes heat which is absorbed by the underlying support target structure. If the heat cannot be transferred away rapidly enough, the temperature of the target member and coating rises. The effective operating life of the phosphor coating may be reduced under continued, excessively high temperature conditions. This places obvious constraints on the sustained, efficient operation of the tube.
To compensate for the dull images as the phosphor coating degrades, some users resort to increasing the intensity of the electron beam, which does temporarily increase the brightness of the resulting image. However, the increased intensity simply compounds the heat problem. Some phosphors, such as blue phosphors, are particularly susceptible to high temperature levels and experience a relatively rapid reduction in their ability to provide images of the desired brightness. Finally, the increased temperature levels may cause the various components of the tube to expand excessively, which in turn, requires undesirable refocussing of the image.
Various structures have been used by those skilled in the prior art to control or reduce the high temperature levels and concomitant deleterious effects experienced in the operation of projection cathode ray tubes. The inventor of the U.S. Pat. No. 4,177,400 suggests that the various metallic components of the structure supporting the target member can provide a path for the conduction of heat away from the target member. Under conditions in the tube requiring the use of a relatively high level intensity electron beam, the transfer of heat by conductivity through the components away from the target member often cannot be accomplished rapidly enough. Thus, the components themselves, especially where bright images over a sustained period are required, reach temperature levels too high for operation without the deleterious effect on the phosphor coating, eventually resulting in having to prematurely replace the tube.
U.S. Pat. No. 3,524,197 to Soule issued Aug. 11, 1970 illustrates the use of a target that comprises the rear surface of the tube and is made from a copper substrate containing a plurality of cooling channels. Water flows through the channels to remove the heat. While the use of a cooling technique, such as described in the U.S. Pat. No. 3,524,197 does transfer heat from the target member, other undesired effects may be introduced. By reducing the mass of the underlying substrate by channels or otherwise providing a hollow construction, the ability of the substrate to uniformly dissipate the heat by conduction from the coating is adversely affected. Such hollow target members introduce undesired temperature gradients within the body of the target. Also the construction of such target members is made much more complicated and expensive.
Still another hollow target coolant system set forth in Japanese Patent Application 55-108,796 published Mar. 3, 1980 describes the use of a heat pipe system based on the premise of liquid that evaporates absorbing (through the heat of vaporization) the excess heat from a hollow target substrate coated with fluorescent material. The problem of removing the heat once it is transferred to the end of the pipe is not fully explained. Such hollow structure in the target member is likely to pose problems identical to those described above.
These and other prior art techniques for cooling and controlling the temperature levels of the target member have not proved to be entirely satisfactory due to the inability to transfer heat rapidly enough away from the target member or requiring expensive and complicated structures to accomplish the transfer. It would be desirable, therefore, to provide a projection cathode ray tube with an improved target assembly that provides increased ability to reduce and efficiently control the operating temperature of the target member, even at relatively high intensity electron beam levels. Additionally, it would be desirable that such an improved target assembly be able to be easily and economically manufactured.